1. Field of the Invention
The present invention relates to a direct methanol fuel cell liquid fuel, to a direct methanol fuel cell cartridge using the liquid fuel of the present invention, and to a direct methanol fuel cell system using the liquid fuel of the present invention. The present invention more specifically relates to a direct methanol fuel cell liquid fuel, which has a reduced total content of components adversely affecting electricity generation efficiency for improving electricity generation performance of a fuel cell.
2. Related Background Art
A polymer electrolyte fuel cell generally has a layered structure in which a proton conductive polymer electrolyte membrane is held between an anode (fuel electrode) and a cathode (oxidizer electrode). The anode and the cathode are each composed of a mixture of: a catalyst having a noble metal such as platinum or an organometallic complex carried on conductive carbon; an electrolyte; and a binder. A fuel supplied to the anode passes through fine pores of the anode, reaches the catalyst, and releases electrons by the action of the catalyst to convert into hydrogen ions. The hydrogen ions pass through the electrolyte membrane provided between the electrodes, reach the cathode, and react with oxygen supplied to the cathode and electrons flowing from an external circuit into the cathode, to thereby produce water. The electrons released from the fuel pass through the catalyst and the conductive carbon carrying the catalyst in the anode, are guided to the external circuit, and flow into the cathode from the external circuit. As a result, in the external circuit, the electrons flow from the anode to the cathode such that electric power is taken out.
Of the fuel cells, a direct methanol fuel cell (DMFC) allowing electricity generation by mere direct supply of a liquid fuel containing methanol and water has particularly attracted attention because of its advantages such as easy and safe handling of the fuel, and simple fuel storage. Further, applications of the DMFC are expected in various fields such as portable electrical devices.
Japanese Patent Application Laid-Open No. 2001-93551 discloses that a fuel storage vessel for DMFC is formed of a removable vessel or a vessel that may be refilled with a liquid fuel, to thereby allow size reduction and long-term drive of a DMFC system.
However, in actual long-term drive of this kind of DMFC system, trace amounts of impurity components in an aqueous methanol solution to be supplied as a fuel accumulate in a fuel cell. The accumulation of the impurity components provides adverse effects such as deterioration of catalyst performance, thereby causing a problem in that an electromotive force of the fuel cell becomes unstable. The problem can be presumably solved by removing the impurity components in the liquid fuel in advance. However, which impurity components in the liquid fuel must be reduced to what extent are hitherto not known for realizing long-term stable drive of DMFC.